US7639363B2 - Electro-optic imaging Fourier transform spectrometer - Google Patents
Electro-optic imaging Fourier transform spectrometer Download PDFInfo
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- US7639363B2 US7639363B2 US11/518,536 US51853606A US7639363B2 US 7639363 B2 US7639363 B2 US 7639363B2 US 51853606 A US51853606 A US 51853606A US 7639363 B2 US7639363 B2 US 7639363B2
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- 239000005262 ferroelectric liquid crystals (FLCs) Substances 0.000 abstract description 3
- 230000001902 propagating effect Effects 0.000 abstract description 3
- 230000003595 spectral effect Effects 0.000 abstract description 2
- 230000003287 optical effect Effects 0.000 description 9
- 230000010287 polarization Effects 0.000 description 8
- 239000000463 material Substances 0.000 description 7
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- 238000005259 measurement Methods 0.000 description 5
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- 239000013078 crystal Substances 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/28—Investigating the spectrum
- G01J3/45—Interferometric spectrometry
- G01J3/453—Interferometric spectrometry by correlation of the amplitudes
- G01J3/4531—Devices without moving parts
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/28—Investigating the spectrum
- G01J3/2823—Imaging spectrometer
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/28—Investigating the spectrum
- G01J3/447—Polarisation spectrometry
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/28—Investigating the spectrum
- G01J3/45—Interferometric spectrometry
- G01J3/453—Interferometric spectrometry by correlation of the amplitudes
- G01J3/4532—Devices of compact or symmetric construction
Definitions
- the present invention relates to a Spectrometer and more particularly to an electro-optic imaging Fourier transform spectrometer comprising a single optical path in which the intensity of light that exits the spectrometer after the light traverses an input polarizer, a series of adjustable birefringent phase retarders, and an output polarizer is simply related by the total optical phase delay to a portion of the frequency spectrum of the light.
- FTS Fourier transform spectrometers
- FTSs require large changes in total optical path length traversed by a beam of electromagnetic radiation. This has typically been accomplished by scanning Michelson interferometers in which one mirror of the interferometer is physically moved to change its length.
- Such an interferometer design has the advantage that a large, continuous band of frequencies can be resolved by scanning large distances with the great precision usually enjoyed by modern mechanical devices.
- interferometers tend to be very large, heavy, slow, have many moving parts, require ultra-precise alignment, and consume relatively large amounts of power to operate.
- the present invention relates to a spectrometer.
- the spectrometer comprises an input polarizer.
- the input polarizer includes an input polarizer center point, an input polarizer axis through the input polarizer center point, and an input polarizer azimuth vector originating on the input polarizer center point.
- the input polarizer azimuth vector points substantially perpendicular to the input polarizer axis.
- the spectrometer also comprises an output polarizer.
- the output polarizer includes an output polarizer center point, an output polarizer axis through the output polarizer center point, and an output polarizer azimuth vector, which originates on the output polarizer center point and points substantially perpendicular to the output polarizer axis.
- the output polarizer output polarizer axis is substantially collinear with the input polarizer axis, thus defining a long axis with an input end proximate the input polarizer and an output end proximate the output polarizer.
- the long axis projects through the input polarizer center point and the output polarizer center point.
- the long axis further defines an input polarizer orientation between the input polarizer azimuth vector and the long axis and an output polarizer orientation between the output polarizer azimuth vector and the long axis.
- the spectrometer also comprises an input quarter wave switch and an output half wave switch.
- the input quarter wave switch is positioned between the input polarizer and the plurality of birefringent phase elements, the input quarter wave switch having a center point and an input quarter wave switch azimuth vector originating on the input quarter wave switch center point and pointing substantially perpendicular to the long axis, thus defining an input quarter wave switch orientation between the input quarter wave switch azimuth vector and the long axis.
- the output half wave switch is positioned between the output polarizer and the plurality of birefringent phase elements, the output half wave switch having a center point and an output half wave switch azimuth vector originating on the output half wave switch center point and pointing substantially perpendicular to the long axis, thus defining an output half wave switch orientation between the output half wave switch azimuth vector and the long axis.
- the spectrometer also comprises a plurality of birefringent phase elements residing between the input polarizer and the output polarizer.
- the birefringent phase elements include a birefringent phase element center point and a birefringent phase element azimuth vector originating on the birefringent phase element center point.
- the birefringent phase element azimuth vector point substantially perpendicular to the long axis, thus defining a birefringent phase element orientation between the birefringent phase element azimuth vector and the long axis.
- An orientation of at least one of the elements is changed, the element being selected from the group consisting of the input polarizer quarter wave switch orientation, the output polarizer half wave switch orientation, and any of the birefringent phase element orientations, allowing the user to substantially reproduce Fourier components of frequency spectra of light passing fully through spectrometer substantially parallel to the long axis.
- the spectrometer further comprises a controller.
- the controller is operably connected with at least one element selected from the group consisting of the input polarizer quarter wave switch, the output polarizer half wave switch, and any of the birefringent phase elements.
- the controller can change the orientation of any element to which it is operably connected.
- each birefringent phase element comprises an achromatic switch and a birefringent phase retarder.
- the birefringent phase retarder is substantially adjacent to the achromatic switch.
- the birefringent phase element is oriented such that the achromatic switch is nearer the output end of the long axis than the birefringent phase retarder.
- the birefringent phase element adjacent to the input polarizer has an achromatic switch with phase retardance of substantially 90 degrees and all other birefringent phase elements have achromatic switches with phase retardances of substantially 180 degrees.
- birefringent phase elements there are an integer N of birefringent phase elements, and for every integer, j, from 0 to N, there is exactly one birefringent phase element with an achromatic switch with phase retardance of substantially 180 degrees and a birefringent phase retarder with phase retardance of substantially 2 j times 360 degrees.
- the orientations of the N birefringent phase elements can be changed to create a substantially binary set of phase delay values.
- the present invention also comprises a method for forming and using the spectrometer described herein.
- FIG. 1 is a block diagram, illustrating an Electro-optic Imaging Fourier Transform Spectrometer with three birefringent phase elements and a controller.
- FIG. 1 also contains an exploded view of one of the birefringent phase elements, showing that the birefringent phase element comprises a birefringent phase retarder and an achromatic switch.
- the present invention relates to spectrometer and more particularly to an electro-optic imaging Fourier transform spectrometer for hyperspectral imaging.
- the following description is presented to enable one of ordinary skill in the art to make and use the invention and to incorporate it in the context of particular applications.
- Various modifications, as well as a variety of uses in different applications will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to a wide range of embodiments.
- the present invention is not intended to be limited to the embodiments presented, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
- any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C. Section 112, Paragraph 6.
- the use of “step of” or “act of” in the claims herein is not intended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.
- Achromatic switch refers to a series of optical components with two special orientations that transform the polarization of light propagating through the material in some set of special directions, the degree of polarization transformation being substantially independent of the wavelength of light over a range determined by the user and the material parameters.
- the achromatic switch comprises at least 2 passive birefringent films or crystals on either side of a twisted nematic or ferroelectric liquid crystal.
- Birefringent refers to a material whose index of refraction depends on polarization. In particular, the index of refraction is different for two, independent, linear polarizations orthogonal to the propagation direction.
- Birefringent phase retarder refers to a birefringent material so oriented that light propagating through the material in some set of special directions and orientations has the phases of its orthogonal polarization components changed with respect to one another by some desired amount.
- Frequency Spectra The term “frequency spectrum” or “frequency spectra” (plural) as used with respect to this invention refers to the function that describes the relative energy density per unit frequency or per unit wavelength in a beam of light.
- Polarizer refers to a material that attenuates orthogonal linear polarizations of light by dramatically different amounts, so that light that interacts with the material becomes substantially polarized in some desired direction after the interaction.
- the spectrometer 100 comprises an input polarizer 102 , including an input polarizer center point 104 , an input polarizer axis 106 through the input polarizer center point 104 , and an input polarizer azimuth vector 108 originating on the input polarizer center point 104 and pointing substantially perpendicular to the input polarizer axis 106 .
- the spectrometer 100 also includes an output polarizer 110 , including an output polarizer center point 112 , an output polarizer axis 114 through the output polarizer center point 112 , and an output 110 polarizer azimuth 116 vector originating on the output polarizer center point 112 and pointing substantially perpendicular to the output polarizer axis 114 .
- the output polarizer axis 114 is substantially collinear with the input polarizer axis 106 , thus defining a long axis 118 with an input end 120 proximate the input polarizer 102 and an output end 122 proximate the output polarizer 110 .
- the long axis 118 projects through the input polarizer center point 104 and the output polarizer center point 112 .
- the long axis 118 further defines an input polarizer orientation 124 between the input polarizer azimuth vector 108 and the long axis 118 and an output polarizer orientation 126 between the output polarizer azimuth vector 116 and the long axis 118 .
- the spectrometer 100 further comprises a plurality of birefringent phase elements 128 residing between the input polarizer 102 and the output polarizer 110 .
- Each of the birefringement phase elements 128 include a birefringent phase element center point 130 and a birefringent phase element azimuth vector 132 originating on the birefringent phase element center point 130 and pointing substantially perpendicular to the long axis 118 , thus defining a birefringent phase element orientation 134 between the birefringent phase element azimuth vector 132 and the long axis 118 .
- An input achromatic quarter wave switch 140 is positioned between the input polarizer 102 and the plurality of birefringent phase elements 128 , the input quarter wave switch 140 having a center point 142 and an input quarter wave switch azimuth vector 144 originating on the input quarter wave switch 102 center point and pointing substantially perpendicular to the long axis 118 , thus defining an input quarter wave switch orientation 146 between the input quarter wave switch azimuth vector 144 and the long axis 118 .
- the input quarter wave switch is variable in four steps: 0, ⁇ , ⁇ /2, and ⁇ /2.
- An output achromatic half wave switch 148 positioned between the output polarizer 110 and the plurality of birefringent phase elements 128 , the output half wave switch 148 having a center point 150 and an output half wave switch azimuth vector 152 originating on the output half wave switch center point 150 and pointing substantially perpendicular to the long axis 118 , thus defining an output half wave switch orientation 154 between the output half wave switch azimuth vector 152 and the long axis 118 .
- the output half wave switch is variable in two steps: ⁇ /2, and ⁇ /2.
- the spectrometer 100 is capable of changing a variety of orientations, non-limiting examples of which include the input polarizer quarter wave switch orientation 144 , the output polarizer half wave switch orientation 152 , and any of the birefringent phase element orientations 134 to substantially reproduce Fourier components of frequency spectra of light passing fully through the spectrometer 100 substantially parallel to the long axis 118 .
- FIG. 1 also shows a controller 136 operably connected with at least one element selected from the group consisting of the input polarizer quarter wave switch 140 , the output polarizer half wave switch 148 , and any of the birefringent phase elements 128 .
- the controller 136 can change the orientation of any element to which it is operably connected.
- FIG. 1 also shows, in an exploded view demarcated by the dashed box, that, in a particular mode, the birefringent phase elements 128 comprise a birefringent phase retarder 140 and an achromatic switch 138 .
- the birefringent phase retarder 140 substantially adjacent to the achromatic switch 138 .
- the birefringent phase element 128 is oriented such that the achromatic switch 138 is nearer the output end 122 of the long axis 118 than the birefringent phase retarder 140 .
- NASA headquarters are located at 300 East Street, Southwest, Washington, D.C.
- the spectral region of this spectrometer will be 1000 to 4000 wave-numbers to allow high-resolution, high-speed hyperspectral imaging applications.
- Due to the use of a combination of birefringent phase retarders and multiple achromatic phase switches to achieve phase delay, this spectrometer is capable of hyperspectral measurements similar to that of the conventional Fourier transform spectrometer but without any moving parts.
- Major NASA applications are the remote sensing of the measurement of a large number of different atmospheric gases simultaneously in the same airmass.
- the reported new technology will result in the development of a high-resolution spectrometer without any moving parts that will provide a substantial improvement in reliability, mission duration, and performance to the next-generation Earth orbiting Fourier transform spectrometers that have been extensively deployed in orbit for atmospheric monitoring. It also promises to be much smaller in size and mass.
- the solution to the above problems is to construct a high-resolution Fourier transform spectrometer that, instead of using a mechanical Michelson interferometer, consists of cascaded birefringent crystals or films for phase delay and achromatic phase switches to achieve a solid-state programmable phase delay without any moving parts. This will represent a substantial improvement in reliability, mission duration, and performance. It also promises to be much smaller in size and mass.
- the EOIFTS is built upon a sequence of the time-delay unit.
- the EOIFTS consists of an input polarizer, a quarter wave achromatic switch, a series of N liquid crystal based electro-optic switches interlaced with a series of passive birefringent phase retarders.
- the basic building block of the system is the unit consisting of a single achromatic half-wave switch between two neighboring passive wave retarders. The principle is that one can select between the sum or difference in total retardation of the wave passing through these two passive wave retarders by rotating the in-between achromatic half-wave switch.
- an achromatic half-wave retarder oriented at zero degrees gives the difference in retardation, while an orientation of 45 degrees gives the sum.
- a long time delay can be achieved that is essential for achieving a high-resolution spectrometer.
- passive retarder thicknesses i.e. 1 wavelength, 2 wavelengths, 4 wavelengths, etc.
- the output of the spectrometer is a periodic representation of the original bandlimited spectrum of input light. This periodicity results from the fact that the spectrometer samples the autocorrelation of the light's electric field to recover the spectrum. Due to the limited number of time-samples, the output spectrum is more accurately described as a smoothed periodic representation of the input. Knowing this, one can consider the input as a single cycle of the resulting periodic output spectrum.
- the total power on the detector is the integral of the input spectrum modulated by the transmission function of the EOIFTS.
- the achromatic quarter wave switch and the last achromatic half wave switch can separate four measurements of the input spectrum, three of which are independent and necessary to fully reconstruct the input spectrum.
- achromatic switches are well-known the art, but, as a concrete example, achromatic half-wave switches in one embodiment of the present invention were constructed by sandwiching a ferroelectric liquid crystal with 90 degree polarization rotation at 1120 nanometers wavelength, the input director oriented at 74 degrees, between two retardation films, each film possessing a full-wave retardance at 600 nanometers wavelength. These achromatic half-wave switches were substantially a half wave in a 1.5 micron band centered near 1.75 microns wavelength.
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US11/518,536 US7639363B2 (en) | 2005-09-07 | 2006-09-07 | Electro-optic imaging Fourier transform spectrometer |
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US11/518,536 US7639363B2 (en) | 2005-09-07 | 2006-09-07 | Electro-optic imaging Fourier transform spectrometer |
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Cited By (1)
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US20180095307A1 (en) * | 2016-10-03 | 2018-04-05 | Xerox Corporation | Hyperspectral imaging system |
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CA2826834C (en) | 2011-02-11 | 2014-07-29 | Canadian Space Agency | Method and system of increasing spatial resolution of multi-dimensional optical imagery using sensor's intrinsic keystone |
US11490037B2 (en) | 2014-10-29 | 2022-11-01 | Palo Alto Research Center Incorporated | Liquid crystal fourier transform imaging spectrometer |
US10760967B2 (en) * | 2014-10-29 | 2020-09-01 | Palo Alto Research Center Incorporated | Liquid crystal fourier transform imaging spectrometer |
US10469771B2 (en) | 2014-10-29 | 2019-11-05 | Palo Alto Research Center Incorporated | Liquid crystal fourier transform imaging spectrometer |
US10564504B2 (en) * | 2017-11-30 | 2020-02-18 | Palo Alto Research Center Incorporated | Liquid-crystal variable retarder using liquid crystal cells of differing thicknesses |
US10175116B1 (en) | 2017-12-29 | 2019-01-08 | Palo Alto Research Center Incorporated | Color filter used with liquid-crystal polarization interferometer |
US10983338B2 (en) | 2017-12-29 | 2021-04-20 | Palo Alto Research Center Incorporated | Exit-pupil expander used distribute light over a liquid-crystal variable retarder |
US10379043B2 (en) | 2017-12-29 | 2019-08-13 | Palo Alto Research Center Incorporated | Measuring path delay through a liquid-crystal variable retarder at non-uniform retardance intervals |
US10663346B2 (en) | 2017-12-29 | 2020-05-26 | Palo Alto Research Center Incorporated | Method and apparatus for transforming uniformly or non-uniformly sampled interferograms to produce spectral data |
CN110009032B (en) * | 2019-03-29 | 2022-04-26 | 江西理工大学 | Hyperspectral imaging-based assembly classification method |
CN110376140B (en) * | 2019-08-14 | 2020-06-30 | 西南交通大学 | Experimental device for be used for detecting full wave band hyperspectral under different electric field environment |
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US6421131B1 (en) * | 1999-07-02 | 2002-07-16 | Cambridge Research & Instrumentation Inc. | Birefringent interferometer |
US6992777B2 (en) * | 2001-11-13 | 2006-01-31 | Adc Telecommunications, Inc. | Birefringent Mach-Zehnder interferometer |
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Patent Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
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US6421131B1 (en) * | 1999-07-02 | 2002-07-16 | Cambridge Research & Instrumentation Inc. | Birefringent interferometer |
US6992777B2 (en) * | 2001-11-13 | 2006-01-31 | Adc Telecommunications, Inc. | Birefringent Mach-Zehnder interferometer |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20180095307A1 (en) * | 2016-10-03 | 2018-04-05 | Xerox Corporation | Hyperspectral imaging system |
US10768497B2 (en) * | 2016-10-03 | 2020-09-08 | Xerox Corporation | Hyperspectral imaging system |
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